1. Field of the Invention
This application relates to the field of vibration analysis and more particularly to the field of performing vibration analysis for the purpose of providing device adjustments that reduce vibrations.
2. Description of Related Art
Rotors which propel helicopters and other propeller-driven aircraft induce vibrations in the structure supporting the rotor. The vibrations occur at frequencies that correspond to the shaft rotation rate and harmonics thereof. The vibrations may result in a structural damage, crew fatigue, and ultimately become one of the factors limiting the maximum forward speed of the aircraft. Similar types of vibrations are produced by fans and compressors and fixed installations as well as by marine propellers.
A primary source of the vibration problem is non-uniform air loads on the blades, although mass imbalance is not uncommon. Aerodynamic anomalies, however, tend to develop recurrently due to blade wear, damage, deformation, etc. The aerodynamic and mass and stiffness distribution anomalies have often been called xe2x80x9ctracking faultsxe2x80x9d, since a primary observable feature of the uneven airloads or mass distribution is it tendency for the blades to flap and/or deflect unevenly, and thus follow different xe2x80x9ctracksxe2x80x9d. The troublesome manifestation of the aerodynamic and mass imbalance, however, is usually the 1/rev and n/rev vibrations and not the track deviations themselves.
It is possible to modify the vibration characteristics of a helicopter by xe2x80x9crotor trimmingxe2x80x9d, which involves adjusting the weight of the blades at the hub, the tab setting at one or more blades, and the adjustment on the pitch rods. However, determining the effect of each of these adjustments may be difficult because the interdependence of the adjustments. This interdependence may be the source of some difficulty with trial and error methods of rotor trimming, which may allow variation of only one type of adjustment at a time. One set of adjustments may be thrown out of kilter by a subsequent step in the process, requiring repetitive adjustments which may or may not converge to an acceptable state.
Some helicopter rotor trim balancing methods rely, at least in part, upon making the track of each blade identical using, for example, known optical methods. Optical methods, however, employ bulky equipment which relies upon an operator in the co-pilot seat and procedures which require considerable flight time. Furthermore, optical methods cannot always xe2x80x9cseexe2x80x9d the blades during a complete revolution and thus cannot be expected to achieve perfect aerodynamic trim. Thus, it is desirable to trim the rotor without having to resort to optical tracking. Fortunately, mechanically balancing the rotors to reduce vibrations often has the desirable effect of improving the rotor track.
Mechanical balancing of rotors with mass imbalance may, in some cases, be performed with a single accelerometer and a shaft-phase reference sensor. However, uneven air loads may not be fully diagnosed and corrected with such a technique. Other techniques used to perform the rotor smoothing function may rely upon optical tracking in conjunction accelerometers. Known rotor smoothing systems, however, process vibration data in such a way that there may be an inherent ambiguity in the interpretation of the signatures. The ambiguity comes about because, in many cases, the number of channels processed simultaneously is inadequate to fully separate translational and rotational acceleration components at a given point. Thus, the motion of the helicopter (and in particular the rotor support) in response to a rotor anomalies may be incompletely specified. Furthermore, some systems may not deduce the corrections needed from the Fourier coefficients related to each anomaly.
U.S. Pat. No. 4,937,758, which is incorporated herein by reference, attempts to address these problems by disclosing techniques that take into account the interdependence of the adjustments. However, the system disclosed therein suffers from drawbacks, including difficulties in adequately relating different types of adjustments to each other and difficulties associated with specifying adjustments that do not take into account acceptable granularity, quantization of the adjustments, and maintenance preferences. Furthermore, it is also desirable to minimize the risk associated with a proposed adjustment where the risk corresponds to the size of the adjustment when compared to the maximum allowable adjustment of that type (i.e., it is more risky to make a large adjustment than a small one). In addition, in some instances it is desirable to be able to correlate risks associated with different types of adjustments (e.g., compare the risk associated with an N ounce hub weigh adjustment with the risk associated with an M degree tab bend).
According to the present invention, determining adjustments for rotating blades includes measuring a plurality of vibration values corresponding to vibrations caused by the blades, providing coefficient of vibration data that corresponds to effects on vibration caused by adjustments to the blades, applying the coefficient of vibration data to the vibration values, and solving for possible values for the adjustments that may be applied to the blades, wherein the possible values for the adjustments are constrained to be greater than a first predetermined value or zero. The adjustments may include at least one of hub weights, blade tabs, PCR adjustments, and blade tip weight adjustments. Measuring vibrations may include obtaining signals from a plurality of accelerometers. Determining adjustments may also include performing an FFT on the vibration data, where the data indicating effects on vibration is provided in the frequency domain. Applying the data to the vibration values may be performed in the frequency domain. The vibration values, the coefficient of vibration data, and the adjustments may all be matrixes. The vibration values may include a first matrix, u, corresponding to measured vibrations and a second matrix, v, corresponding to desired vibrations. Solving for possible values for the adjustments uses the equation v=u+Xxc2x7a, where a represents the adjustment and X represents the coefficient of vibration data. Solving for possible values of the adjustments may include replacing X with Qxc2x7R, where QTxc2x7Q equals the identity matrix, I and R is an upper triangular matrix. Solving for possible values of the adjustments includes evaluating ∥QTxc2x7u+Rxc2x7a∥.
According further to the present invention, determining adjustments for rotating blades includes sorting sets of possible adjustments for the blades based on a predetermined criteria and electing a set that is constrained so that all of the adjustments of the set are greater than a first predetermined value or zero. Sorting sets of possible adjustments may include measuring a plurality of vibration values corresponding to vibrations caused by the blades, providing coefficient of vibration data that corresponds to effects on vibration values caused by sets of possible adjustment applied to the blades, applying the coefficient of vibration data to the vibration values to predict an effect on vibration values caused by each of the sets of possible adjustments, and sorting the sets of possible adjustments according to the effect on vibration values for each of the sets. Measuring vibrations may include obtaining signals from a plurality of accelerometers. The predetermined criteria may be a measure of an amount of vibrations caused by a set of adjustments. Determining adjustments for rotating blades may include deselecting sets of adjustments that result in vibrations greater than a predetermined amount. The adjustments may include at least one of hub weights, blade tabs, PCR adjustments, and blade tip weight adjustments.
According further to the present invention, computer software that determines adjustments for rotating blades includes executable code that receives a plurality of vibration values corresponding to vibrations caused by the blades, executable code that accesses coefficient of vibration data that corresponds to effects on vibration caused by adjustments to the blades, executable code that applies the coefficient of vibration data to the vibration values, and executable code that solves for possible values for the adjustments that may be applied to the blades, where the possible values for the adjustments are constrained to be greater than a first predetermined value or zero. The plurality of vibration values may be provided by signals from a plurality of accelerometers.
According further to the present invention, computer software that determines adjustments for rotating blades includes executable code that receives sets of possible adjustments for the blades sorted based on a predetermined criteria and executable code that selects a set that is constrained so that all of the adjustments of the set are greater than a first predetermined value or zero. The computer software may also include executable code that receives a plurality of vibration values corresponding to vibrations caused by the blades, executable code that accesses coefficient of vibration data that corresponds to effects on At vibration values caused by sets of possible adjustment applied to the blades, executable code that applies the coefficient of vibration data to the vibration values to predict an effect on vibration values caused by each of the sets of possible adjustments, and executable code that sorts the sets of possible adjustments according to the effect on vibration values for each of the sets. The vibration values may include signals from a plurality of accelerometers. The predetermined criteria may be a measure of an amount of vibrations caused by a set of adjustments. The computer software may also include executable code that deselects sets of adjustments that result in vibrations greater than a predetermined amount. The adjustments may include at least one of hub weights, blade tabs, PCR adjustments, and blade tip weight adjustments.
According further to the present invention, an apparatus that determines adjustments for rotating blades includes means for receiving a plurality of vibration values corresponding to vibrations caused by the blades, means for accessing coefficient of vibration data that corresponds to effects on vibration values caused by sets of possible adjustments applied to the blades, means for applying the coefficient of vibration data to the vibration values, and means for solving for possible values for the adjustments that may be applied to the blades, where the possible values for the adjustments are constrained to be greater than a first predetermined value or zero. The adjustments may include at least one of hub weights, blade tabs, PCR adjustments, and blade tip weight adjustments. The means for measuring vibrations may include means for obtaining signals from a plurality of accelerometers. The means for receiving a plurality of vibration values may include a first input. The means for accessing coefficient of vibration data may include a second input. The predetermined value may be provided by a third input.
According further to the present invention, an apparatus that determines adjustments for rotating blades includes means for receiving sets of possible adjustments for the blades that are sorted based on a predetermined criteria and means for selecting a set that is constrained so that all of the adjustments of the set are greater than a first predetermined value or zero. The apparatus may also include means for receiving a plurality of vibration values corresponding to vibrations caused by the blades, means for accessing coefficient of vibration data that corresponds to effects on vibration values caused by sets of possible adjustment applied to the blades, means for applying the coefficient of vibration data to the vibration values to predict an effect on vibration values caused by each of the sets of possible adjustments, and means for sorting the sets of possible adjustments according to the effect on vibration values for each of the sets. The vibration values may be provided by signals from a plurality of accelerometers. The predetermined criteria may be a measure of an amount of vibrations caused by a set of adjustments. The apparatus may also include means for deselecting sets of adjustments that result in vibrations greater than a predetermined amount. The adjustments may include at least one of hub weights, blade tabs, PCR adjustments, and blade tip weight adjustments.